In measuring the dynamic characteristics of an analog to digital converter, i.e. an ADC, a series of outputs of the ADC are acquired while the input signal is continuously changed. Analysis of the output data can be carried out by operating the ADC with a clock of frequency f.sub.CLK while providing the ADC with cycles of a wave usually a sine wave, having a frequency f.sub.IN.
When the measurement of an m-bit ADC is done as described above, the number of all the possible code words is 2.sup.m, and it is preferable to have all of the code words appear at the output of the ADC during the measuring process. If the input signal is a sine wave and the Analog to Digital-conversion is to be done at uniform intervals, at least .pi. * 2.sup.m-1 number of conversions of different bit combinations are to be done in one cycle of the input signal but, actually more Analog to Digital-conversions are done.
Because of the limitation of the conversion speed, however, it is not possible to perform all the necessary conversions during a single cycle of the input signal. Thus, the required Analog to Digital-conversions are done over a multiplicity of cycles of the input sine wave.
When the necessary Analog to Digital conversions are to be done over n cycles of a sine wave input signal, the ratio, f.sub.IN :f.sub.CLK should not be the ratio between two simple integers; otherwise a plurality of points on the input sine wave of the same phase would be repeatedly sampled for Analog to Digital-conversion during the n cycles. It would therefore be impossible to use all the possible code words as the digital outputs of the ADC. In FIG. 1, for example, f.sub.IN :f.sub.CLK =1:4, in which case only 3 code words could be evaluated since code words for the phases 0 and .pi. are of the same value. At most there would be four, even if the phase of the clock is slightly shifted. In FIG. 2, however, f.sub.IN :f.sub.CLK =5:16, the output code words of the ADC can be evaluated at 16 different phases of the input sine wave over 16 clock cycles. Actually, it is necessary to take more data points, and it is preferable to make a slight phase difference between the input sine wave and the clock, typically a quarter of the period of the clock.
The measurement of dynamic characteristics of an ADC as described above has been done with a configuration such as shown in FIG. 3. In this figure, oscillators 42 and 44 are provided to respectively generate an input sine wave signal and a clock. ADC 24, which is the device under test, Analog to Digital-converts this generated input sine wave at the timing of the clock 44 and stores the results sequentially in a memory 26. After providing the necessary number of date samples to the memory 26, any required evaluation is done by reading them out using a data processing device or the like, not shown. As one example of such an evaluation, consult, How to Evaluate the Performance of Waveform Digitizers, Denshi Kagaku, July, 1981, pp. 18-23, or Japanese patent laid-open No. Showa 62-38618, entitled Apparatus for Measuring Dynamic Characteristics, assigned to Yokogawa-Hewlett-Packard Ltd. Because the evaluation itself does not directly relate to the substance of the present invention, no further explanation will be given here.
When the measurement is executed with the arrangement shown in FIG. 3, the relationship between the oscillation frequencies of the oscillator for the sine wave input signal to the ADC 24 and the oscillator 44 for the clock signal may arbitrarily be set. However, some jitter exists between these frequencies since the sine wave input signal and the clock signal are generated by mutually independent oscillators. Because it is quite difficult to remove this jitter, it interferes with the measurement of the dynamic characteristics of a high speed ADC. Furthermore, because it is impossible to control the phase relationship between these two signals with this arrangement, the measurement starts at a point of different phase on the sine wave input for each measurement. For example, assume that the first measurement produces the output codes to the memory 26 corresponding to the series of points shown in FIG. 2. The next measurement might start at the point shifted by .DELTA. from the starting point of the first measurement. Because the measurement conditions vary from measurement to measurement, the measurement method of the prior art lacks repeatability.